As areal density of hard disks increases, bit dimensions on hard disks correspondingly decrease, leading to increased testing time when testing each disk completely. Conventional media testers for head based certification typically use a flying head with a read sensor. For high resolution testing, the read sensor needs to be narrow at about 200 to 400 nm, resulting in longer testing times. For a read sensor width ranging from 150 to 350 nm, if only one certification head is used, a complete or full measurement of just one disk having a radius of 0.7 to 1.6 inches at 7200 rpm can take from 8 to 22 minutes, as shown in FIG. 1. As bit dimensions further reduce, read sensors will correspondingly narrow, resulting in even longer testing times for full disk scanning.
In a typical media fabrication process, head based media certifiers are used to detect defects on disk media. As an alternative to head based certification, optical based certification using a Laser Doppler Vibrometer (LDV) provides shorter testing times as shown in FIG. 2 due to use of a larger beam spot that allows each disk to be completely tested more quickly. However, this has a disadvantage of reduced resolution due to the larger spot size and furthermore, the detection capability of LDV-based detection is less than that for head based certifiers. Optical certifiers are therefore not usually used alone for defect detection of hard disks. Head based certifiers are more commonly used although optical certifiers are gaining importance as increasing disk densities will lead to longer certification times required by head based certifiers.